I am assuming that both the cars you part exchanged were on HP. If so, they did not belong to you or to the garage which took them from you - they belonged to the finance company.
Even if you did not tell the garage they were on HP (& I gather from your post that you did) the garage had a liability to do an HP check and - if they didn't do one - the garage did not obtain good title to the cars. However, if the garage then sold them to a private individual (not another dealer) who was not aware of the situation, that person would get good title and the car could not be reclaimed from them. Nonetheless it would be a good idea to get from DVLA (or for the finance companies to do this) the identity etc. of the new owner because if it is a dealer they may not have good title. (I believe you would have to show good cause to get the information and pay DVLA a small fee - ask them.)
I suspect the HP agreements may have something in the wording saying the car must not be disposed of without the finance company's agreement. I assume you did not yourself contact the finance company on either occasion. What arrangement did you make with the garage? Was it left that they would pay off the rest of the HP? If so, is this in writing or did you just let them tell you verbally they would do it and accept that?
If no-one told the finance companies at all what had been done then I'm very much afraid that - so far as they are concerned - you are still liable because you entered into the contracts with them and they are still valid. You would then have to make a claim against the liquidator of the garage (or possibly against the owner in person if it was a sole trader), but if it was insolvent you would be unlikely to get any money.
You really need some specialist advice about this - which probably means paying to see a solicitor. Or you could take all the papers to your local CAB and ask them to see whether their